1. Technical Field
THIS INVENTION relates to peroxide bleaching of pulp.
Pulp bleaching is the chemical process that alters the colour giving components within the pulp to give it a higher brightness. In a conventional peroxide bleaching process, alkali is required to react wit the peroxide to generate a perhydroxy ion (HOOxe2x88x92), the bleaching agent responsible for the bleaching reaction.
2. Background
Typically the alkali used in peroxide bleaching is sodium hydroxide, but it has a number of disadvantages associated with its use. The main problem being a competing reaction that occurs between the caustic and the pulp. This reaction is the yellowing reaction, and as the name suggests, causes the pulp to yellow in colour.
A process that describes peroxide bleaching which utilizes magnesium oxide as a sole alkaline source has been described in International Publication No. WO96/41917. It was found that the brightness levels obtained using MgO were very close to that which is achieved by conventional bleaching using NaOH. A number of benefits have been found in the use of MgO as the sole alkali source including cost savings over the caustic system, buffering action, no yellowing reaction and improved handling conditions as MgO is not a dangerous product.
More specifically International Publication No. WO96/41917 referred to a process wherein the pulp is bleached in the presence of hydrogen peroxide for a maximum period of 180 minutes to achieve an ISO brightness of 65 in regard to freshly prepared pulp characterized in that the magnesium oxide is utilized as MgO particles having a particle size of 5-500 microns and a particle surface area of between 20-60 m2/g. A preferred particle size is less than 75 microns and more preferable 30-50 m2/g.
However, the process described in International Publication No. WO96/41917 was unduly restricted in relation to the maximum ISO brightness of 65 as well as the maximum bleaching time of 180 minutes. These factors impaired commercial viability of this bleaching process especially when high 180 brightness levels greater than 65 were required in certain situations
Surprisingly, it has now been discovered that higher brightness levels can be more efficiently achieved by monitoring the concentration of transition metals in the source of magnesium oxide and/or by the choice of chelating agent used in the bleaching process.
Therefore, in one aspect of the present invention, there is provided a process for peroxide bleaching of pulp using magnesium oxide or magnesium hydroxide as a substitute for caustic soda Wherein the pulp is bleached in the presence of hydrogen peroxide characterized in that the concentration of iron present in the magnesium oxide or magnesium hydroxide and the pulp is maintained less than 20 ppm and/or the concentration of manganese is maintained less than 10 ppm.
It has now been ascertained that if the concentrations of manganese and iron exceed the above levels then the efficiency of the bleaching process may be adversely affected because such relatively high concentrations of iron or manganese by reacting with the peroxide will decrease the peroxide concentration to below the optimum concentrations required.
In regard to this aspect of the invention, it will be appreciated that iron and/or manganese will be present both in the magnesium oxide and the pulp and thus it Is therefore necessary to monitor the total concentration of iron and/or manganese present.
It is well known that transition metal ions may be removed by a washing step prior to bleaching of pulp and this is usually achieved by the use of DTPA In the case of International Publication No. WO96/41917 a further dose of DTPA is added to the pulp during bleaching. However, it could not have been predicted that by minimizing the Mn and/or Fe levels in the MgO, and therefore maintaining the levels below the above concentrations that higher target ISO brightness could be more efficiently achieved by the specific process described in International Publication No. WO96/41917.
In another aspect of the invention, there is provided a process for peroxide bleaching of pulp using magnesium oxide or magnesium hydroxide as a substitute for caustic soda wherein the pulp is bleached in the presence of hydrogen peroxide characterized in that a bleach solution comprising the pulp and the magnesium hydroxide or magnesium oxide has a chelating agent which is selected from a group consisting of a chelating agent having a phosphinic acid moiety, a chelating agent not having a phosphinic acid moiety and silicate in accordance with the following criteria:
(a) when the concentration of Mn in the bleach solution is greater than 10 ppm, the chelating agent is chosen from the chelating agent having a phosphinic acid moiety and a chelating agent not having a phosphinic acid moiety;
(b) when the Fe concentration is greater than 20 ppm, the chelating agent is silicate; and
(c) if silicate is to be avoided as a chelating agent in the case when the Fe concentration is greater than 20 ppm, the chelating agent has a phosphinic acid moiety.
The particle size of the MgO particles or Mg(OH)2 particles is less than 500 microns and preferably less than 75 microns. The surface area of the Mg(OH)2 or MgO particles is preferably 20-60 m2/g and more preferably 30-50 m2g.
The chelating agent having a phosphinic acid moiety is preferably DTPMPA (i.e. diethylene triamine pentamethylene phosphonic acid). The chelating agent not having a phosphinic acid moiety may be selected from DTPA (diethylene triamine penta acetic acid), EDTA (ethylene diamine tetra acetic acid) and HEDTA (hydroxethylene diamine tetra acetic acid).
In relation to the use of silicate as a chelating agent, preferably use is made of an alkali metal silicate such as sodium silicate or potassium silicate.
It is also the case that criteria (a) and (b) above may apply together, i.e. when the concentration of iron in the bleach solution is greater than 20 ppm and the concentration of manganese in the bleach solution is greater than 10 ppm, both chelating agents (i.e. the chelating agent having the phosphinic acid moiety and silicate) may be added.
The dosages of MgO that may be utilized in the process of the invention are preferably 0.3-2% based on the weight of the pulp.
The amount of hydrogen peroxide that may be utilized in the process of the invention is from 1-6% based on the weight of the pulp.
To achieve maximum efficiency, the MgO particles are preferably added to the pulp in the form of a powder or slurry prepared in situ.
Preferably the MgO is added to the pulp simultaneously with the peroxide or prior to the addition of the peroxide.
Also, to achieve maximum efficiency, the Mg(OH)2 should be manufactured by precipitation using MgCl2 as opposed to hydration of MgO.
It has now been demonstrated if either aspect of the invention as described above is followed, then a target ISO brightness of above 65 may be achieved, i.e; of the order of 70-80. Thus, the target ISO brightness may be from 55-80. The bleaching time of 180 minutes referred to in International Publication No. WO96/41917 may also be exceeded if desired. Thus, the bleaching time may be unlimited.
The purpose of this work is to determine.
(i) the effect of metal ions (Fe and Mn) contained in MgO on the bleaching efficiency of MgO based peroxide bleaching; and
(ii) the most effective metal chelating agent for Fe and Mn in an MgO based bleaching process.
Preferably the pulp before the bleaching step is washed with 0.1-0.5% v/w DTPA based on the pulp. This concentration of chelating agent may also be used in the bleach solution.